A plane waveguide type laser has a structure in which both the upper and lower surfaces of a thin plate-like laser medium extending in a traveling direction of laser light are sandwiched by clads whose refractive index is lower than that of the laser medium, and has a structure of making the laser medium also operate as a waveguide. Because this plane waveguide type laser has a thin waveguide thickness and a high excitation density, a large gain can be acquired and therefore an efficient oscillation operation can be implemented even when a laser medium having a small cross section for stimulated emission is used. In addition, by extending the waveguide in a width direction, scaling of the output power can be carried out while the excitation density is held at a predetermined value. On the other hand, there is an issue of making the laser oscillate in a linear polarization, which is required at the time of wavelength conversion.
Therefore, conventionally, a planar waveguide laser device that implements laser oscillation having a linear polarization, as shown in, for example, patent reference 1 has been proposed. This planar waveguide laser device is comprised of a laser medium having birefringence, and a clad material having a refractive index ranging between the refractive indexes for two polarized light rays which are traveling within the laser medium along an optical axis and whose planes of vibration are perpendicular to each other (TE (Transverse Electric) polarized light (polarized light whose plane of vibration is perpendicular to a plane formed by the c axis and the optical axis which is the traveling direction of the laser light and exists in a plane including the optical axis, and which is also called ordinary light), and TM (Transverse Magnetic) polarized light (polarized light whose plane of vibration exists in the plane formed by the c axis and the optical axis, and which is also called extraordinary light)). Because a material has a refractive index ranging between the refractive index for the TE polarization of the laser medium and the refractive index for the TM polarization of the laser medium is used as the clad, either the TE polarized light or the TM polarized light does not satisfy the total reflection condition, and therefore the laser oscillation of either one of the polarizations which satisfies the total reflection condition becomes possible.